Vibration translating device



Nov. 7, 1933.

P. B. FLANDl-:Rs Er AL 1,934,416

VIBRATION TRANSLATING DEVICE Filed oct. 2o, 195o 2 sheets-sheet 1/A/f/EA/TORS P. E. FLANERS HL 6, HA/QR/SN @y @MM TTOR/VEV NV- 7 1933 P.B. FLANDl-:Rs Er AL 1,934,416

VIBRATION TRANSLATING DEVI CE Filed Oct. 20, 1930 2 Sheets-Sheet 2 s- 27c y RESPONSE /V DEC/EELS l lo vli|||| l l||||||| llltlall 30 50 IOO 500|000 5033i)l |0000 20600 FREQUENCY Ml CYCLES PEI? SECO/V0 @ff/@WWWTTOR/VEV Patented Nov. 7, 1933 VIBRATION TRANSLATING DEVICE Paul B.Flanders, East Orange, N. J., and Henry C. Harrison, Port Washington, N.Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application October 20, 1930. SerialNo.-489,846

e claims. (ci. 179-111) ll'his invention relates to vibrationtranslating devices of the electrostatic type and more particularly totransmitters comprising a stationary back electrode and a diaphragm ofstretched material disposed in close face-to-ace relation, the diaphragmbeing adapted to be vibrated to vary the capacity of the device.

While the invention is not necessarily limited in its application totransmitters, since a similar structure may also be used in othertranslating devices, it is particularly applicable to transmitters forsound pick-up purposes in high quality systems and will, therefore, bedescribed with rer"- erence to this application for purposes ofillustral" is the general object of the invention to sirntheconstruction and improve the operation these devices.

e improvement attained, according tc the tion, consists in providingrelatively low cost, a device which is capable of responding at ui'formly efficiency to a much greater range frequencies than theinstruments available heretofore. It is comparatively easy to design adevice or this kind which has any one of these desirable characteristicsbut when it is sought to combine them in a single structure, a number ofdifficulties are encountered which will be better understood. from abrief review of the prior art.

In Patent "1,333,744 to C. Wente, March 16, 192() there is disclosed atransmitter in which the sound responsive element is a thin metaldiaphragm closely spaced with respect to a plate member which forms theother electrode of the condenser. A heavy clamping ring surrounds thiselectrode and serves to tension the diaphragm and thereby raise itsnatural frequency to a point near or above the upper limit of theWorking range. Since a very high tension is required, the clamping ringmust be rather large and it there- :tore materially increases the sizeof the device.

The thermophone calibration, which is commonly used for the purpose or"indicating its response, shows that an instrument of this kind operatesat practically a constant eciency over a very Wide range of frequenciesbut more recent investigations have disclosed that there are additionalfactors which considerably modify the response of the device in actualuse. The more important of these factors are the increase in response incertain parts of the range due to the air resonating in the` cavityformed by the clamping ring and the increase due to the disturbance inthe sound field caused by placing the instrument in it.

This latter effect is discussed at some length by Stuart Ballantine inan article Effect of Diffraction Around the Microphone in SoundMeasurements inthe Physical Review, vol. XXXII, December 1928. Brieflystated, if the instrument is small as compared with the wave length ofthe sound it is a case of ordinary Laplacian flow of air past anirregular object and the pressure on the diaphragm is practically thatin the undisturbed sound ileld while if the diaphragm is of infiniteextent the pressure on it Will be double that of the undisturbed soundeld for all frequencies due to the reection effect. The condensertransmitters commonly used heretofore have been ot' the order of 3inches in diameter which is approximately equal to the quarter Wavelength of a i 1,100 cycle wave and it is found in accordance with theabove theory that the incident pressure on the diaphragm of such atransmitter varies from normal for frequencies below 1,1@0 cycles tosubstantially twice normal around 2,500 cycles, which results in giving6 db. gain in response for-all irequencies above this latter value.

A third factor which causes the actual response to dii'er from thatindicated by the thermophone calibration may be termed the phase loss.which is due to the variation in pressure across the diaphragm occurringwhen waves of length less than or comparable to the diameter of thediaphragm approach the transmitter from directions other than directlyin iront of the instrument. For instance, when the transmitter is facingthe sound source in a live room there will be considerable phase lossdue to the Waves deflected from the walls of the room.

ln accordance with the general features of this invention the range ofuniform frequency response of these devices is materially extended bysubstantially eliminating the cavity in which resonance occurs andmaking the instrument very small so as to minimize the phase loss andcause the pressure doubling eiect to occur much higher in the frequencyscale where it will be less objectonable.

Heretofore, these results have not been attainable because of certainpractical dimcultles. In order to stretch thediaphragm and maintain itat the required tension the stretching and clamp-I ing frame must bemade rather bulky, thereby causing both the cavity and the pressuredoubling effects to occur in the important speech range. Otherconsiderations, such as loss of efnciency due to unavoidable deadcapacity and other factors, limit the possible reduction in the d'ameterof the diaphragm itself, and hence the improvement in response which canbe gained by dit,

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merely making the present structures smaller is also rather limited. Ifthe necessity for the frame is eliminated by using an unstretcheddiaphragm, it is impossible to keep the response reasonably constantover a wide range of frequencies without making the device veryinecient. Since the output of any condenser transmitter is very small,even small losses in eiiciency are objectionable.

In order, therefore, to make a satisfactory device of this kind, whichis very small, it is essential that its efficiency be kept up. This canbe done only by keeping the diaphragm very thin and light and designingthe electrodes of the device to give maximum generated voltage. In fact,the requirements for the daphragm are such that stretching is necessaryfor otherwise, with a diaphragm of the proper physical properties, theclose and accurate spacing of the electrodes cannot be maintained inservice. The tensionng of the diaphragm is also necessary in order thatits resonant frequency will occur high enough in the frequency scale toavoid resonance peaks in the working range. In View of all these factsit will be evident that the object of the invention can'be attained onlyby a novel structure.

According to one important feature of the invention, a very smalltransmitter is made possible by tensioning the diaphragm by externalmeans which form no part of the completed instrument, and using aclamping ring on the instrument of only sucient size to maintain thenecessary tension. This in itself not only practically eliminates thecavity resonance but also materially reduces the necessary size of thedevice.

Another important feature of the invention is the proportioning of theback electrode to give maximum generated voltage with minimum inactivecapacity. The novel arrangement and proportioning of the electrodesaccording to the invention together with the prestretched diaphragmfeature, provides a construction which is capable of covering a widerfrequency range at a higher efciency as compared with presentAlternatively, a transmitter designed in accordance withthe invention tocover only the range now commonly used may be made considerably moreeflicient than present devices.

By suitably combining these features as will be more fully explained, ithas been found practical to reduce the diameter of these transmitters toconsiderably less than one-third of those now commonly used and at thesame time to reduce the cost very materially.

The several features of the invention will be better understood from astudy of the following detailed description of a specific structure madein accordance with the principles discussed above and the drawings inwhich Fig. 1 is an enlarged sectional view of a transmitter according tothe invention;

Fig. 2 is an assembly of such a transmitter and its associatedamplifier;

Fig. 3 is an exploded view of the transmitter of Fig. 1; and

Fig. 4 is a series of response curves illustrating the improved resultsobtainable according to the invention.

Referring now to Fig. 2, the transmitter 1 is attached by a threadedconnection to the tube 2 which at its other end leads into the vacuumtube amplifier 3. The function of the tube is to separate thecomparatively large bulk of the amplier from the transmitter so as toavoid disturbance to the sound field. From the foregoing discussion itwill be evident that the tube should not be substantially larger indiameter than the transmitter and that its length will necessarilydepend upon the relative size of the transmitter and the amplier. It 1's found, for instance, that with a transmitter of about 1 inch diameterand an amplifier of 3 inch diameter, a tube of 6 inches to 8 inches inlength is satisfactory. Since it is desirable to keep the capacitybetween the leads from the transmitter to the amplifier a minimum, thetube may be used as one lead and the other may consist of a wire 16suitably held in the center of the tube by insulating spacers 17 asindicated.

The detailed construction of the transmitter is shown more clearly inFig. 1 in which the metal casing 4 carries an internal steel ring 5which forms a seat for the back electrode struc-, ture 6. This structureconsists of a steel electrode or plate 7 and a steel member 8 secured byscrew 20 on opposite sides of an insulator 9 which supports theelectrode on the ring 5. The diaphragm l0 is secured between the steelclamping ring 11 and the surface l2 of the casing 4 to give the properspacing of 1 mil between the diaphragm and the face 18 of the electrode7. One convenient way of making this adjustment accurately is toassemble the device leaving out the washer 19 which is of thicknessequal to the desired electrode spacing, lap the surfaces 12 and 18 untilthey are in exactly the same plane and then insert the washer 19 whennally assembling the instrument. Dowel pins 21 and 22 are provided tohold the several parts of the assembly in their proper relation and aslot 23 is cut in the insulator 9 to provide an air leakage path so thatthe air pressures on the opposite sides of the diaphragm will not becomeunequal due to temperature changes. The paper washers 35 and 36 are usedin the assembly to distribute the pressures uniformly over the surfaceof insulator 9.

A sheet of diaphragm material which is preferably duralumin of 1 milthickness is tensioned in a suitable frame (not shown) and the clampingring 1l is secured in place before the diaphragm 10 is cut out of thesheet so that, as previously pointed out, the ring need be only ofsufcient size to maintain the tension. The insulator 9 is preferably ofPyrex glass or other suitable dielectric material which will not warpand thereby destroy the accuracy of the electrode spacing. The electrode7 has a circular face 18 adjacent to the diaphragm but is cut away atthe back so as to leave a spider-like construction of arms 13 radiatingfrom the center. In this manner it is made very stable in its mountingand, due to the relatively small area of the electrode presented to thecasing and the relatively large air space surrounding the arms theinactive capacity between the electrode and the casing is very small ascompared with the active capacity between the electrode and thediaphragm. In accordance with well known principles as set forth inPatents 1,333,744 and 1,722,347 to Wente and Patent 1,456,538 toCrandall, the necessary damping is obtained by the displacement of theair between the diaphragm and the back electrode but excessive dampingat high frequencies is prevented by grooving and perforations in theface of the electrode. In this case, however, this construction has beensimplified by the use of a single circular groove 14 with openings 15 inthe relatively thin portions of the electrode between the arms 13.

It is well understood in the art that the effective response of acondenser transmitter depends not only upon the design of thetransmitter itself but also on the characteristics of the amplifier withwhich it is to be used. The ideal arrangement would be to have an ampli-Iier of infinite input impedance, but since there are design factorswhich limit the maximum obtainable amplifier impedance the usefulresponse is necessarily slightly less than the generated voltage wouldindicate. It has been found advisable in the design of `thesetransmitters to disregard this fact to proportion the parts for maximumgenerated voltage and to use an amplifier of the highest input impedanceobtainable rather than to base the design on the assumption that anamplier of any given impedance will be used.

For maximum generated voltage in any condenser transmitter it has beenfound that there is a definite relation between the optimum electrodesizes and certain other design factors. This may be expressed by sayingthat the ratio of the radius of the back electrode, a, to the radius ofthe free portion of the diaphragm, R, should be as follows:

where m is equal to the free radius of the vdiaphragm in centimeterssquared, divided by the product of twice the spacing of the electrodesin centimeters and the inactive capacity of the instrument in absoluteelectrostatic units.

A study of the series of curves shown on Fig. 4 gives a betterunderstanding of the general statements in the first part of thespecification and of the improved results attained in accordance withthe invention. These curves are plotted with a common logarithmicfrequency scale as abscissa, with separate ordinate scales (in decibelsin accordance with the usual practice in calibrating devices of thiskind). The zero level in each case is the same and represents anarbitrary value chosen for purposes of comparing the various curves.

Curve 25 is the thermophone calibration curve of a typical instrument ofthe prior art and indicates that the response is uniform for allpractical purposes up to about '7000 cycles since variations of 3 db. orless are scarcely detected by the average ear. Curves 26 and 27 show thecavity resonance and pressure doubling effects already discussed whichalter the response of the instrument as commonly used. The actualresults obtained when the'transmitter is facing the sound source in openair (or in a comparatively dead or heavily damped room) are thereforemore nearly indicated by curve 28 which is the sum of curves 25, 26 and27. However, if the transmitter is located at 90 to the sound source oris used in a live room where a very large percentage of the energyapproaches the instrument in the plane of the diaphragm, even curve 28is by no means an accurate indication of the actual response obtained.

In the 90 position the instrument, which is usually comparatively thintransversely to the plane of the diaphragm, is a small obstacle forsound waves over the part of the important frequency range so that the.transition eiect occurs so much higher in the scale that it may notmaterially .alter the response over the range for which it is' commonlyused. The cavity resonance .'dinarily would be unchanged but there wouldbe a loss at high frequencies due to phase differences in the waves forvarious parts of the diaphragm as shown by curve 29. Hence, for thisextreme condition the response is indicated by the thermophonecalibration plus the cavity resonance, minus the phase loss as shown bycurve 30. It is evident that the results obtained under the conditionsof either of the curves 28 or 30 are very different from those indicatedby the thermophone calibration of curve 25. For instance, the lattercalibrationl indicates that the f response does not reach the,arbitrarily chosen zero level until about 9000 cycles is reached,whereas under the conditions of curve 30 the zero level is actuallyreached at 6000 cycles. While it is true that the curves 28 and 30represent extreme conditions and that for some applications the actualresponse is somewhat more uniform than these curves would indicate, itwill be apparent from the foregoing explanation that the response ofthese devices of the prior art diiers Widely with the conditions underwhich they are used. Consequently, a certain amount of frequencydistortion in the upper range is unavoidable withthese devices and, asindicated by the curves, the very high frequencies are deficient orentirely absent in the output of such instruments.

The characteristics of two types of transmitters made in accordance withthe present invention are designated 31 and 32. These devices are ofessentially the same construction except that the former uses adiaphragm tensioned to about 16,000 lbs. per square inch whereas in thelatter the tension is about 40,000 lbs. per square inch. As is wellunderstood in the art, this results in a higher efliciency for theformer but the range of the device is accordingly less than can beobtained at' a somewhat lower efficiency with the greater tension. Asalready stated the cavity resonances in these new devices are entirelynegligible and are therefore not considered. Curves 33 and 34 show thepressure doubling and phase loss eiects respectively from which thebroken line curves indicating the variations from the thermophonecalibration are plotted for the curves 31 and 32 in accordance with thelegend on theV drawings. thermophone curves 31 and 32 withthe curve 25the material improvement in frequency range From a "comparison oftheused, it will be remembered that the pressure crease above that rangedue to the increasing phase loss.

Present high quality systems are intended effectively to translatefrequencies up to 5,000 or 6,000 cycles but from the foregoingexplanation of the curves of Fig. 4 it will be seen that reasonableuniformity of response near the upper end of this range can be obtainedwith present devices only by using them under conditions which minimizethe irregularities due to the various effects discussed. If, in applyingthe principles of the present invention to any particular case, thisrange is considered sufficient, it is of course unnecessary to make thedevice as small as possible. With the improved clamping arrangement ofthe invention instruments having electrodes of the same size as theprior art devices (which are ordinarily of about 3 inch outsidediameter) may be made of 2 inch diameter or less and it is found thateven this reduction in size gives a very decided improvement in theupper portion of the characteristic.

The instruments used for illustration, however, have an outside diameterof about .9 inch with a free diaphragm diameter of about .7 inch. Itshould be understood, however, that if the relation of the Equation (l)is maintained and proper attention given to the mechanical difficultiesinvolved in manufacturing, still smaller instruments may be made whichwill give even better results than those indicated by the curves 31 and32.

The invention therefore is not intended to be limited to instruments ofany particular size or characteristic except as dened in the followingclaims.

What is claimed is:

1. In combination, a vibration translating device small enough to beplaced in a sound field Without substantially distorting it forimportant sound frequencies, an amplifier and rigid means ofsubstantially the same diameter as the device holding the device and theamplier in spaced relation and constituting an electrical connectionbetween them.

2. In combination, a vibration translating device small enough to beplaced in a sound iield Without substantially distorting it forimportant sound frequencies, an amplifier, rigid means comprising ametallic tube of substantially the diameter of the device forming both amechanical and an electrical connection between the amplifier` and thedevice and a conductor mounted Within the tube completing the electricalconnection.

3. In combination, a vibration translating device not exceeding one inchin diameter comprising a stretched diaphragm and a plate in closeface-to-face spaced relation, a housing supporting the diaphragm, theplate and clamping means for maintaining the tension of the diaphragm,an amplifier and rigid means having a diameter substantially equal tothat of the device forming a mechanical and an electrical connectionbetween the device and the amplifier.

4. In combination, a vibration translating device of the order of oneinch in diameter, comprising a back electrode and a highly tensionedydiaphragm in close spaced relation, and a tubular member ofsubstantially the same diameter as the device supporting the device andconstituting an electrical connection to the diaphragm.

5. In a vibration translating device of the order of one inch indiameter, the combination with a housing, a highly tensioned diaphragmand a back electrode supported by the housing in close spaced relation,of clamping means of sufficient bulk to maintain the tension of thediaphragm, but of such small dimensions that it is incapable oftensioning the diaphragm.

6. A vibration translating device of the order of one inch in diameter,comprising a housing, a back electrode supported by the housing, ahighly tensioned diaphragm over o ne end of the housing, and clampingmeans adapted solely for maintaining the tension of the diaphragmsecured to the housing through the diaphragm and being of such smallbulk and configuration as to be incapable of producing the tension.

PAUL B. FLANDERS. HENRY C. HARRISON.

